In recent years, there has been an increased need for a power generation method that takes the global environment into consideration, and technical development on microbial power generation has been in progress. Microbial power generation is a method of generating power by taking out electrical energy obtained when microbes assimilate organic substances.
Generally, in microbial power generation, microbes, organic substances to be assimilated by the microbes, and an electron transport medium (electron mediator) are made to coexist in a negative electrode chamber in which a negative electrode is disposed. The electron mediator enters the bodies of microbes, receives electrons which are generated when the microbes oxidize the organic substances, and passes the electrons to the negative electrode. The negative electrode is electrically connected through an external resistor (load) to a positive electrode. The electrons passed to the negative electrode move through the external resistor (load) to the positive electrode, and are passed to an electron acceptor in contact with the positive electrode. As a result of the movement of electrons, a current flows between the positive electrode and the negative electrode.
In microbial power generation, since the electron mediator takes electrons directly from the bodies of microbes, the theoretical energy conversion efficiency is high. However, the actual energy conversion efficiency is low, and improvement in power generation efficiency is required. Under these circumstances, in order to improve power generation efficiency, various studies and developments are under way regarding the material and structure of electrodes, the type of electron mediator, the selection of the microbial species, and the like (for example, Patent Literature 1 and Patent Literature 2).
Patent Literature 1 describes that a positive electrode chamber and a negative electrode chamber are separated by an alkali ion conductor composed of a solid electrolyte, each of the inside of the positive electrode chamber and the inside of the negative electrode chamber is kept at pH7 using a phosphate buffer (buffer), and by blowing air into the phosphate buffer (cathode solution) in the positive electrode chamber, power generation is performed.
Patent Literature 2 describes that a porous body is placed as a positive electrode plate so as to be in contact with an electrolyte membrane separating a positive electrode chamber from a negative electrode chamber, air is passed through the positive electrode chamber, and air and a solution is brought into contact with each other in the voids of the porous body. (Hereinafter, a positive electrode in which air is passed through a positive electrode chamber and oxygen in air is used as an electron acceptor in such a manner may be referred to as an “air cathode” in some cases).
In a microbial power generation device using an air cathode, a cathode solution is not required. Furthermore, air is simply passed through the positive electrode chamber, and aeration into the cathode solution is not required, which is advantageous.
Conventionally, in order to improve power generation efficiency in a microbial power generation device using an air cathode, studies have been conducted on
1) the mediator in the negative electrode (for example, Patent Literature 3),
2) pH adjustment in the negative electrode chamber,
3) the type of positive electrode catalyst and the catalytically active component support method,
4) the shape of the positive electrode, and the like.
    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2000-133326    Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2004-342412    Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2006-331706
In a conventional microbial power generation device, power generation efficiency is small at 50 to 150 W/m3 (per m3 of negative electrode), and further improvement in power generation efficiency is desired.